1. Field of the Invention
The present invention relates to a method of reporting a Channel Quality Indicator (CQI) in a wireless communication system. More particularly, the present invention relates to an apparatus and method for reducing an amount of CQI to be delivered from a receiving end to a transmitting end.
2. Description of the Related Art
In general, a wireless communication system uses Adaptive Modulation and Coding (AMC) to improve spectral efficiency by adaptively changing a modulation order and an error correction code with respect to a channel between transmitting and receiving ends. Further, scheduling is performed by considering a channel state between a Base Station (BS) and a Mobile Station (MS) to improve a system throughput when the channel is allocated to the MS. For this purpose, a Channel Quality Indicator (CQI) estimated in the receiving end needs to be delivered to the transmitting end.
In broadband Orthogonal Frequency Division Multiple Access (OFDMA), the AMC and the scheduling are performed for each subband by utilizing a characteristic of a frequency selective channel. A subband generally denotes a basic unit for maintaining a channel quality characteristic. In this case, a per-subband CQI needs to be delivered from the receiving end to the transmitting end, and thus a signaling overhead caused by CQI delivery is increased. As a result, channel resources are wasted.
Accordingly, there is a need for an effective CQI transmission method capable of reducing the signaling overhead while transmitting a CQI sufficient to effectively perform the AMC and the scheduling.
FIG. 1 is a flowchart illustrating a process of generating a time-domain differential CQI in a conventional receiving apparatus.
Referring to FIG. 1, the receiving apparatus estimates a per-subband CQI with respect to each transport layer in step 101, and generates a broadband CQI according to the estimated per-subband CQI in step 103. The broadband CQI may be a representative CQI for a whole system band. Further, the broadband CQI may be a representative CQI for a band excluding subbands in which CQIs are individually delivered for scheduling, or may be a representative CQI for a subset of the band.
In step 105, with respect to each transport layer, the receiving apparatus reads the per-subband CQI and the broadband CQI from a memory in which the CQIs are stored. Herein, the read per-subband CQI and broadband CQI are those most recently delivered to a transmitting end to which a reference signal for encoding a differential CQI is transmitted. In step 107, the receiving apparatus compares the per-subband CQI and the broadband CQI, which are estimated in steps 101 and 103, and the per-subband CQI and the broadband CQIs which are read in step 105, and then obtains difference values thereof. In this case, if the per-subband CQI and the broadband CQI, which are most recently delivered to the transmitting end with respect to each transport layer, do not exist in a subband, a reference value of the subband can be replaced with either a broadband CQI value or a selected CQI value that is obtained when the subband is last selected.
In step 109, the receiving apparatus quantizes the difference value between the per-subband CQI and the broadband CQI, encodes the quantized difference value, and converts the encoded difference value into a bit-stream. In step 111, the receiving apparatus converts a differential CQI, which is converted into the bit-stream, into a transmission symbol, and transmits the converted transmission symbol to the transmitting end. When the difference value between the per-subband CQI and the broadband CQI is quantized and encoded to generate the bit-stream, mapping of Table 1 or Table 2 below is used. A scheme having a constant resolution, as shown in Table 1, is generally used. A scheme having a multiple resolution, as shown in Table 2, is used when a range of the differential CQI is large. In the selection of mapping, the mapping can be adaptively changed according to a channel characteristic. Thereafter, the procedure of FIG. 1 ends.
TABLE 1Differential CQIBit-stream−81000−71001−61010−51011−41100−31101−21110−111110000010001200103001140100501016011070111
TABLE 2Differential CQIBit-stream510007100191010111011−61100−81101−101110−12111100000100012001030011−40100−30101−20110−10111
FIG. 2 is a flowchart illustrating a process of generating a spatial-domain differential CQI in a conventional receiving apparatus.
Referring to FIG. 2, the receiving apparatus estimates a per-subband CQI with respect to each transport layer in step 201, and generates a broadband CQI according to the estimated per-subband CQI in step 203.
In step 205, for the encoding of the spatial-domain differential CQI, the receiving apparatus selects a reference transport layer for the per-subband CQI and the broadband CQI. In step 207, the receiving apparatus compares a per-subband CQI and a broadband CQI of the selected reference transport layer and a per-subband CQI and a broadband CQI of another transport layer, and thus obtains difference values thereof.
In step 209, the receiving apparatus quantizes the difference value of the per-subband CQI and the broadband CQI, encodes the quantized difference value, and converts the encoded difference value into a bit-stream. In step 211, the receiving apparatus converts a differential CQI, which is converted into the bit-stream, into a transmission symbol, and transmits the converted transmission symbol to a transmitting end. Thereafter, the procedure of FIG. 2 ends.
A signaling overhead caused by the spatial-domain differential CQI is larger in comparison with the case of the time-domain differential CQI. This is because at least one transport layer does not transmit a differential CQI with respect to the selected per-subband CQI and the selected broadband CQI. The increased overhead of the spatial-domain differential CQI can be reduced when the time-domain differential CQI is applied for the reference transport layer.
As described above, an amount of the signaling overhead can be reduced in the aforementioned method employing the differential CQI. However, the amount of signaling overhead cannot be expected to be decreased in the case of the time-domain differential CQI since a correlation value of a time-domain CQI is decreased in proportion to the speed of an MS, which results in the increase of a differential CQI range. In the case of the spatial-domain differential CQI, if a CQI correlation between transport layers is small, an overhead reduction effect is smaller in comparison with a method employing the time-domain differential CQI. In addition, in a case where a Forward Error Correction (FEC) output is allowed to pass a plurality of transport layers so that a correlation increases between transport layers, channel adaptability is decreased, and thus there is a problem in that maximization of transmission efficiency achieved through CQI transmission is limited. Therefore, a need exists for an improved apparatus and method for reporting a CQI indicator.